As a method of converting an alternating-current voltage into a direct-current voltage, in general, two methods explained below are known. In a first method, a diode bridge circuit and a smoothing capacitor are used. The diode bridge circuit full-wave rectifies an alternating current from an alternating-current power supply. The smoothing capacitor smoothes a direct current after the full-wave rectification.
In the first method, irrespective of whether the alternating-current voltage is plus or minus, an electric current flows in a series circuit of two diodes. At this point, in the two diodes, a power loss equivalent to a product of electric currents respectively flowing through the diodes and forward voltages of the diodes occurs.
In a second method, a power factor improvement converter (PFC) is interposed between the diode bridge circuit and the smoothing capacitor in the first embodiment. The power factor improvement converter controls an electric current flowing to the alternating-current power supply to be a sine wave shape and control the electric current to be equal to a voltage phase of the alternating-current power supply.
In the second method, as in the first method, in the full-wave rectification, since an electric current flows in the series circuit of the two diodes, a power loss occurs. In addition, since an alternating-current alternately flows in a field effect transistor (FET) and a diode configuring the power factor improvement converter, a further loss occurs.
In the power factor improvement converter, an output voltage has to be set higher than an input voltage because a waveform of an input current needs to be formed in a sine wave shape. However, a voltage necessary for a load is not always a voltage higher than the input voltage. In that case, a step-down converter is connected to a post stage of the power factor improvement converter. A voltage boosted by the power factor improvement converter is stepped down to a desired voltage. A loss also occurs in the step-down. The entire power converting apparatus is configured by three stages of AC-DC conversion, DC-DC (step-up) conversion, and DC-DC (step-down) conversion. Power conversion efficiency appears as a product of conversion efficiencies of the stages. For example, if efficiency per one stage is 0.95, the power conversion efficiency of the three stages is 0.95×0.95×0.95=0.86. That is, even in excellent conversion having efficiency of 95%, the power conversion efficiency drops to 86% in three-stage connection. In this way, even if the respective conversion efficiencies are high, the conversion efficiency is markedly deteriorated if the power converting apparatus is configured in multiple stages.
Recently, there is an increasing demand for power saving of electronic apparatuses. At the same time, it is also an essential condition that current harmonic noise is not emitted to prevent an adverse effect on an external environment. Therefore, it is requested to achieve both of improvement of conversion efficiency of a power converting apparatus that supplies electric power to a load and a function of suppressing current harmonics.
On the other hand, as a method of converting a direct-current voltage into an alternating-current voltage, there is a method of converting a direct-current voltage into an alternating-current voltage using a pulse width modulation (PWM) inverter circuit. With this method, an electric current corresponding to pulse width can be fed to an alternating-current power supply side. Therefore, by controlling the pulse width in synchronization with the alternating-current voltage on the alternating-current power supply side, it is possible to generate an alternating-current waveform same as a voltage waveform on the alternating-current power supply side. However, in this method, in a process for generating the pulse width, a loss occurs in a switching element configuring an inverter and power conversion efficiency is deteriorated. This method has only an inverter function for converting the direct-current voltage into the alternating-current voltage. Therefore, to additionally provide a conversion function from the alternating-current voltage into the direct-current voltage in the opposite direction, a separate circuit for converting the alternating-current voltage into the direct-current voltage has to be prepared.
As explained above, to realize the bidirectional power conversion between the alternating-current power supply and the direct-current power supply, the circuit for converting the alternating-current voltage into the direct-current voltage and the circuit for converting the direct-current voltage into the alternating-current voltage need to be separately provided. Moreover, control for operating any one of the two circuits and stopping the other is necessary. Therefore, the circuits are made redundant and costs of the power converting apparatus increase. The power conversion efficiency of the power converting apparatus is low. Further, the power converting apparatus tends to be increased in size and weight.
JP-A-2011-147277 is an example of the related art.